RESEARCH/REVIEW ARTICLE Sessile macro-epibiotic community of solitary ascidians, ecosystem engineers in soft substrates of Potter Cove, Antarctica Clara Rimondino,1 Luciana Torre,2 Ricardo Sahade1,2 & Marcos Tatia´ n1,2 1 Ecologı´a Marina, Facultad de Ciencias Exactas, Fı´sicas y Naturales, Universidad Nacional de Co´ rdoba, Av. Ve´ lez Sarsfield 299, (5000) Co´ rdoba, Argentina 2 Instituto de Diversidad y Ecologı´a Animal, Consejo Nacional de Investigaciones Cientı´ficas y Te´ cnicas/Universidad Nacional de Co´ rdoba and Facultad de Ciencias Exactas, Fı´sicas y Naturales, Av. Ve´ lez Sarsfield 299, (5000) Co´ rdoba, Argentina Keywords Abstract Sessile macro-epibiont; ascidian; Antarctica; ecosystem- engineer. The muddy bottoms of inner Potter Cove, King George Island (Isla 25 de Mayo), South Shetlands, Antarctica, show a high density and richness of macrobenthic Correspondence species, particularly ascidians. In other areas, ascidians have been reported to Clara Rimondino, Ecologı´a Marina, Facultad play the role of ecosystem engineers, as they support a significant number of de Ciencias Exactas, Fı´sicas y Naturales, epibionts, increasing benthic diversity. In this study, a total of 21 sessile macro- Universidad Nacional de Co´ rdoba, Av. Ve´ lez epibiotic taxa present on the ascidian species Corella antarctica Sluiter, 1905, Sarsfield 299, (5000) Co´ rdoba, Argentina. Cnemidocarpa verrucosa (Lesson, 1830) and Molgula pedunculata Herdman, 1881 E-mail: [email protected] were identified, with Bryozoa being the most diverse. There were differences between the three ascidian species in terms of richness, percent cover and diversity of sessile macro-epibionts. The morphological characteristics of the tunic surface, the available area for colonization (and its relation with the age of the basibiont individuals) and the pH of the ascidian tunic seem to explain the observed differences. Recent environmental changes in the study area (increase of suspended particulate matter caused by glaciers retreat) have been related to observed shifts in the benthic community structure, negatively affecting the abundance and distribution of the studied ascidian species. Considering the diversity of sessile macro-epibionts found on these species, the impact of environmental shifts may be greater than that estimated so far. In coastal marine environments, the availability of predation or physical abrasion by water movement, can substrate is one of the main factors influencing the dis- also determine the development of epibiotic communi- tribution and abundance of macrobenthic sessile organ- ties (Ward & Thorpe 1991). However, epibiosis depends isms. Obtaining a suitable substrate for recruitment is an mostly on the epibiont/basibiont species interaction. important event in the life cycle of any sessile benthic A high degree of specialization in epibiotic communities organism (Connell & Keough 1985; Wahl 1989). To opti- has been detected related to the morphological and phys- mize space, some species have developed the ability to icochemical features of the basibiont, that is, shape, sur- establish themselves on secondary substrates consisting face area, texture (Sebens 1991; Harder 2008) and its of other organisms, a phenomenon known as epibiosis antifouling substances (Nylund & Pavia 2005). In the (Harder 2008; Wahl 2009). This can contribute to gen- particular case of ascidians, some chemical characteristics, erating complex three-dimensional structures and in- such as vanadium concentration (Odate & Pawlik 2007; creased diversity in benthic assemblages (Gutt & Schickan Koplovitz et al. 2009), production of secondary metabo- 1998; Barnes 2001). lites (McClintock et al. 2004) and acid pH (Stoecker 1978, Various factors may influence the intensity of epibiosis, 1980), may help to prevent epibiosis. especially seasonality, latitude and depth (Wahl 1989; Ecosystem engineers are organisms that directly, or Barnes 1994; Vasconcelos et al. 2007). Other biological indirectly, control the availability of resources to other and physical processes, such as competitive interactions, life forms by causing changes in physical states, that is, Polar Research 2015. # 2015 C. Rimondino et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial 3.0 1 Unported License (http://creativecommons.org/licenses/by-nc/3.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Citation: Polar Research 2015, 34, 24338, http://dx.doi.org/10.3402/polar.v34.24338 (page number not for citation purpose) Sessile macro-epibiotic community of solitary ascidians C. Rimondino et al. modifying or even increasing habitat heterogeneity the muddy bottom of the inner cove at a depth of 30 m. (Jones et al. 1994; Wright & Jones 2006). Ascidians often Immediately after sampling, they were photographed and act as ecosystem engineers and enhance local diversity by then fixed and stored in 2.5% formaldehyde in seawater for providing living habitat for some groups of epibionts, such analysis. The tunic pH was determined on fresh individuals as algae and sponges (Monteiro et al. 2002; Castilla et al. collected during January 2013 (see below). Corella antarctica 2004; Voultsiadou et al. 2007). has a flattened ovate body, with a cartilaginous tunic, and a In Potter Cove, the surrounding Fourcade Glacier has smooth surface except in the dorsal area, where it can have been significantly retreating over the last two decades some shallow grooves. Large individuals may reach 24 cm in (Ru¨ ckamp et al. 2011). This has led to changes in hy- length (Tatia´n et al. 1998). The species C. verrucosa is spherical drographical parameters, such as an increase in suspended or more or less cylindrical in shape. This ascidian has a re- particulate matter (Schloss et al. 2012). These may have sistant, rough tunic due to the presence of conical papillae caused the changes observed in macro-epibenthic com- ending in fine, transversely arranged spines, more abundant munity structure, including the decline of solitary ascidian in the area of the siphons but less developed in very large populations and their replacement by other taxa, such specimens. Larger individuals can reach 18 cm in length as pennatulids (Sahade et al. 2008), which have greater (Kott 1969). Molgula pedunculata has an ovoid or rectangular capacity to cope with higher suspended particle concen- erect body, supported by a peduncle of variable length, trations (Torre et al. 2012), but which are not suitable reaching a total length of 30 cm (Herdman 1882). The tunic substrata for epibionts. is translucent and fairly flexible, naked or slightly downy, Although it has been suggested that solitary ascidians especially in the peduncle, with a smooth surface. are important as a substrate for sessile macro-epibionts in Potter Cove (Tatia´n et al. 1998), no studies have so far been performed to understand the composition of their Identification of sessile macro-epibiotic taxa sessile epibiotic communities. Different taxa attached to the tunics of individuals The aim of this study is to describe and compare the (basibionts) of the three ascidian species were carefully diversity of sessile macrobenthic taxa living on the outer observed under a CZM4 trinocular microscope (Labomed, tunic of three of the most conspicuous solitary ascidian Los Angeles, CA, USA) and identified (Ha¨ussermann & species inhabiting soft bottoms at Potter Cove. The results Fo¨ rsterra 2009). Vouchers of the examined material are will make it possible to evaluate sessile macro-epibiotic stored at the Museo de Zoologı´a, Universidad Nacional de community changes in the study area and in other nearby Co´rdoba. Antarctic coastal ecosystems, in a context of shifts mediated by the global change. Estimation of percent cover of sessile Materials and methods macro-epibionts and data analyses For every basibiont, richness was estimated as the number Study area and sampling of different sessile macro-epibiont taxa. Percent cover Potter Cove, where the Argentine Carlini (formerly Jubany) was assessed after Sahade et al. (1998) using a grid of 100 Station and the ArgentineÁGerman Dallmann labora- uniformly distributed points, superimposed on a picture tory are situated, is an inlet of the larger Maxwell Bay, at corresponding to an ascidian individual. Each point of King George Island (Isla 25 de Mayo), South Shetlands, the grid was considered as 1% cover. Taxa present, but Antarctica (62814?S, 58838?W). The cove is 4 km long that were not underlying any point, were considered a and 2.5 km wide and has a maximum depth of 100 m 0.5% cover. Differences in the richness and percent cover (Schloss et al. 2012). The inner cove, characterized by of different taxa of sessile macro-epibionts on the three soft bottoms, is dominated by suspension feeders, mainly ascidian species were assessed by one-way analyses of solitary ascidians (Sahade et al. 1998). variance (ANOVA) at a significance level of 5% with a Individuals of the most abundant solitary ascidian species post-hoc Bonferroni test. Data normality and variance were collected haphazardly during February 2004. These homogeneity were tested by Shapiro-Wilk and Cochran’s species, which can reach a large size, were: Corella antarctica C tests, respectively. (C. eumyota: Alurralde et al. 2013), n10 (8 to 13 cm in The relative abundance of higher sessile epibiotic taxa length); Cnemidocarpa verrucosa, n9(3.5to10cminlength)